Field of the Invention
The invention relates to a hydraulic tappet in a valve actuating mechanism, and more particularly to a hydraulic tappet for a valve train in an internal combustion engine.
Description of the Prior Art
A hydraulic tappet in a valve actuating mechanism of an internal combustion engine is known. German Pat. No 1 033 459 discloses an example of the above.
The known hydraulic tappet automatically compensates, in response to lubricating oil pressure in the tappet, for any play or lash in the valve actuating mechanism of the internal combustion engine. Such known hydraulic tappets with automatic length compensation include a tappet cylinder actuated by a cam and a tappet piston arranged slidingly and spring loaded therein, constructed to actuate a valve directly or indirectly. The tappet piston contains a reservoir chamber permanently loaded by the lubricating oil pressure and is movable in the direction of play compensation by a pressure chamber connectible to said reservoir chamber under certain conditions.
During the transmission of movements and forces between cam and rocker lever or valve, the tappet piston is supported through the oil column enclosed in the pressure chamber against the tappet cylinder when a check valve within the tappet is closed and thus the tappet constitutes a virtually rigid connection.
In the case of an increase of play in the valve actuating mechanism, the length of the hydraulic tappet is increased by the spring loaded tappet piston being urged away from the bottom of the tappet cylinder so that a vacuum is produced in the pressure chamber which in conjunction with the lubricating oil pressure chamber causes the check valve to open and lubricating oil to flow out of the reservoir chamber into the pressure chamber until the existing play has been eliminated.
The compensating hydraulic tappet is also capable of being shortened where there is a reduction of play in the valve actuating mechanism. Oil can escape or leak from the pressure chamber at the peripheral gap between tappet piston and tappet cylinder until the necessary reduction of play has been compensated. The oil leaks from the pressure chamber due to powerful compressive forces generated by the cam and valve and a rapid buildup in oil pressure.
Although the known hydraulic tappets in valve actuating mechanisms of internal combustion engines achieve considerable advantages such as the reduction of the noise level and the avoidance of valve play adjustment operations, they exhibit drawbacks. These drawbacks include a limitation of the maximum camshaft speed, restrictions to gentle cam contours, and stringent demands as to freedom from trapped air in the lubricating oil. The individual points which produce these disadvantages are listed hereinbelow.
In order to permit the passage of the lubricating oil from the reservoir chamber into the pressure chamber, which is necessary in the case of an increase in play, it is necessary for the pressure of the lubricating oil supply to overcome the resistance of the check valve. The oil system must maintain a minimum pressure. The minimum pressure must be maintained at idle speed and maximum oil temperature which dictates a correspondingly large capacity lubricating oil pump and a low resistance design of the lubricating oil passages in the engine.
The arrangement of the check valve in the pressure chamber, which is dictated by design considerations, leads to a relatively large volume in the pressure chamber so that the static rigidity of the oil column c=BA.sup.2 /V (where B=compressibility module of the oil, A=effective piston area, and V=volume of pressure chamber) diminishes and thus, negatively influences the natural frequency and the transmission function of the valve actuating mechanism.
In order to ensure the automatic length compensation of the hydraulic tappet in the case of play reduction, relatively large gap widths are necessary at the peripheral gap between tappet piston and tappet cylinder, since otherwise it would be impossible to obtain a corresponding rapid shortening of the hydraulic tappet in the warming-up phase of the engine when immediate power demands lead to a rapid heating of the engine and hence to rapid length increases in the valve actuating mechanism and increases in the volume of the oil in the pressure chamber.
However, the large gap widths which are necessary for the above stated reasons have a disadvantageous influence on the dynamic rigidity of the hydraulic tappet specifically at higher oil temperatures and higher forces. In addition to the diminished static rigidity of the oil column due to a great volume of the pressure chamber, the dynamic rigidity is diminished due to a constant tendency to shorten the hydraulic tappet by leakage at the peripheral gap between the tappet piston and cylinder defined by equation:
Q=ps.sup.3 b/12nl
(Q+leakage flow, P=pressure in pressure chamber, S=radical gap, b=gap width, 1=gap length, and n=viscosity of oil).
A further problem in the known hydraulic tappets occurs when the engine is switched off and switched back on after a period of time. The known hydraulic tappets are designed to allow air to be sucked from the reservoir into the pressure chamber if the hydraulic tappet is stopped in the open position. The introduction of air into the pressure chamber renders the hydraulic tappet inoperable for a period of time upon the next operation of the engine which results in objectionable rattling and shortened valve strokes. Further difficulties with known hydraulic tappets result from the fact that when the valve speed limit is attained at which the rocker lever and tappet are no longer pressed against the cam by the valve spring, a recoil of the oil column in the pressure chamber results in the suction of oil and hence to an undesirable pumping-up of the hydraulic tappet which leads to the destruction of the tappet. Natural vibrations of the hydraulic tappet due to insufficient rigidity can also lead to the destructive pumping-up.